The present invention relates to a printer.
Printers have various motors such as a paper feed motor for driving a feed roller that conveys print paper or a print object and a carriage motor for driving a carriage having a print head. DC motors are widely used as such motors to reduce noise. Printers having DC motors are equipped with an encoder composed of a scale having marks or slits disposed at specified intervals and a sensor that senses the marks or slits of the scale to output given signals to control the positions and speeds of the DC motors.
For example, to control a paper feed motor, printers have a disc-shaped scale having multiple slits arranged at specified intervals and a sensor constructed to sandwich each slit between a light-emitting device and a light-receiving device. This type of scale is constructed to rotate with a feed roller. This type of sensor generally outputs two signals with a phase difference of 90° (for example, refer to Japanese Patent Publication No. 2001-232882). The motor is controlled by sensing changing points of the levels of the two signals output from the sensor. For example, the rotational position and speed of the motor are sensed using the two signals output from the sensor, and the rotational speed of the motor is controlled by PID control on the basis of the sensed rotational position and speed.
In order to improve print quality, more accurate control is required for motors mounted to printers. For more accurate control, encoders have to output signals with higher resolution. There may be two methods for outputting higher-resolution signals from encoders: a method of increasing the diameter of the disc-shaped scale while maintaining the intervals of the slits and a method of decreasing the interval of the slits while maintaining the diameter of the scale.
However, printers that need to be compact cannot have a large-diameter scale. To provide the space for the scale, the mechanical structure of the printers becomes complicated. In contrast, narrowing the interval between slits makes it difficult to manufacture the scale itself.